I have a rather significant vaccuum leak in my 350. I have looked at the obvious, but I'm not sure where to look next. Any advice?

 

There are several different ways to track down a vacuum leak, but tracking it down can be, on the one hand, time consuming; and on the other you may hit it right off.

With the engine idling you can use a spray bottle filled with plain tap water and squirt it around all the possible locations. If you get the water near the leak, the water will get sucked in and the idle will drop. If the leak is really bay you may seeb white smoke coming from the tailpipe.

Sometimes you can hear the leak as a hissing sound.

Others use a propane torch UNLIT and when the gas gets sucked in the idle will change.

Direct the gas as all the possible leak places including where the injectors enter the manifold

All hoses should be checked too; especially the underside. If hard and brittle they will leak. Heat causes that over time. The cruise control and fuel vacuum canister are often culprits. Don't forget the underside of the throttle body.

Start at the front of the engine on, say, the driver's side and check everything all the way to the firewall. Then switch over to the passenger side and repeat. Take your time and be thorough.

Keep us posted.

Jake



------------------
1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

How do you know you have a vacuum leak?

 

Well, when I put a vaccuum gauge on, it had no vaccuum. That was my first tip off. Next to that, I can't adjust my idle down. Also, my brand new brakes are almost completely out.
Today I was permatex-ing the front and back of my intake manifold and putting on a new carb gasket when I realized that there were four completely open bolt holes between my carb and intake manifold. How much would that be contributing to the leak?

 

Do you mean there are four bolts missing out of your intake manifold?

 

Yup, there are four bolt holes totally open. The intake etc does not exactly match up with the carb and apparently these holes were overlooked. I just thought of the fact that they could be false holes, though, so I'm going to go home and check if they go all the way through.

 

lol, easy there tiger. A vacuum leak will not cause you to have *no* vacuum. I've got a quarter says you hooked to a ported vacuum source.
Beyond that, we don't even know what kind of car you have. If we are to assume it's a B4C(your name) and a 350, and i think the majority of those are 90-92 SD cars. So, yes, a vacuum leak could be causing a high idle. However, what are you calling a high idle? Perhaps your tach is off and everything is working normally w/n spec, and thus of course you can't get the idle down, cause the computer opens the IAC to compensate when you close the throttle plates.

So let's start at the beginning, what is your problem and what have you checked?
...ed


------------------
Ed Maher - Moderator @ The Carb Board
92 Z28 Convertible - Quasar blue / Tan top
LB9 4L60 GU2 G80 - stock, soon to be sleeper
-=ICON Motorsports=-

- Definitely prototypes, high powered mutants of some kind. Too weird to live, too cool to die

 

Sorry guys, I was in a rush the night I posted this, and neglected to give you any background. My car's a '91 B4C with a 350 that has less than 400 miles on it, but was beat to hell, and I had to fix just about 70% of everything in the engine compartment. The car finally runs, but naturally has some quirks yet to be fixed. The car did not have zero vaccuum, but it was so low that it might as well have been non-existent. After filling in the open bolt holes in my manifold, however, I now am ecstatic to report that I have 17 inches of vaccuum. So at the present, the vaccuum is fixed, as well as the idle, but I am having trouble with timing now. I have pulled the distributor many times and adjusted bit by bit. It runs very smooothly at idle, but knocks just a tad. There is also a pre-ignition pinging. The main problem is when I put it in gear, though. The car flat out dies. I posted a question specifically about this if any of you have more advice in this area. Thanks for helping.